Not Applicable.
The present application relates to cryocatheters and wands, i.e. to catheters and wands which are used to treat tissue by cooling contact. Such implements, henceforth generically referred to herein as xe2x80x9ccryocathetersxe2x80x9d or simply xe2x80x9ccathetersxe2x80x9d have an elongated body through which a cooling fluid circulates to a tip portion which is adapted to contact and cool tissue. In general, cryocatheters may be used to lower the temperature of tissue, such as cardiac wall tissue, to an extent such that signal generation or conduction ceases and allows one to map or confirm that the catheter is positioned at a particular lesion or arrhythmia conduction site. They may also be configured for ablation treatment, to cool the tissue to a much lower level at which freezing destroys the viability of the tissue, and, in the case of cardiac tissue, permanently removes it as a signal generating or signal conducting locus. Such devices are also useful for destruction of tissue at other body sites, such as the ablation of tumorous, diseased, precancerous or congenitally abnormal tissue in various vessel or organ systems.
Cryocatheters may be adapted for endovascular insertion, or for insertion along relatively confined pathways, for example through a body lumen, or through a small incision to and around intervening organs, to reach an intended ablation site. As such, they are characterized by an elongated body through which the cooling fluid must circulate, and a tip or distal end portion where the cooling is to be applied. The requirement that the coolant be localized in its activity poses constraints on a working device. For example when the catheter contact must chill tissue to below freezing, the coolant itself must attain a substantially lower temperature. Furthermore the rate of cooling is limited by the ability to supply coolant to and circulate it through the active contact region, and the efficacy of the contact region itself is further limited by geometry and physical properties that affect its ability to conduct heat into the tissue. The achievable rate of cooling may be impaired by warming due to residual circulation, and it also changes depending upon the effectiveness of thermal contact, e.g. upon the contact area and contact pressure between the catheter and the tissue, as well as being influenced by ice accumulations or other artifacts or changes due to the freezing process itself. Moreover, it is a matter of some concern that proximal, adjacent or unintended tissue sites should not be exposed to harmful cryogenic conditions. These somewhat conflicting requirements make the implementation of an effective cryocatheter a complex matter.
One approach has been to provide a phase change coolant which is pumped as a liquid to the tip of the catheter and undergoes its phase change in a small chamber located at the tip. The wall of the chamber contacts adjacent tissue directly to effect the cooling or ablation treatment. Such a device can treat or achieve a relatively high rate of heat energy transfer. Moreover, by employing a phase change refrigerant which may be injected at ambient temperature along the body of the catheter and undergo expansion at the tip, the cooling effect may be restricted to the localized treatment region surrounding the tip portion of the device. The dimensions of catheter construction, particularly for an endovascular catheter, require that the phase change coolant be released from a nozzle or tube opening at a relatively high pressure, into a relatively small distal chamber of the catheter. After the fluid expands in the distal chamber and cools the walls, it is returned through the body of the catheter to a coolant collection system, preferably in the form of a recirculation loop.
Catheters or treatment wands of this type for endovascular or endoscopic use generally have a relatively symmetrical profile such as a pencil-like cylindrical shape or a bullet-like shape, and they may include various steering mechanisms for curving the tip to urge it against the tissue which is to be treated. Fluoroscopic visualization may be used both for positioning the catheter tip initially and for observing the progress of ice formation. However, the cooling effect is extremely local, relying on thermal conduction through contact, and the method of visualization may involve a plane projection, or may otherwise lack precision or resolution. Also, because of the extreme temperatures involved, the tip of the catheter may freeze to tissue which it contacts, preventing any further adjustment or repositioning of the tip once cooling has started. Moreover, if the catheter construction involves any degree of asymmetry, the fluoroscopic representation may be insufficient to determine the effective area of contact or expected cooling profile in surrounding tissue.
It has been proposed, for example in U.S. Pat. No. 5,667,505, that because of the impossibility of measuring the tip-to-tissue contact, one instead measure the temperature of a heat exchanger, and utilize a standard heat capacity measurement to develop appropriate control signals. Furthermore, with these and other types of ablation catheters, various detection systems have been proposed for determining the degree of contact or the extent of heating. See, for example, U.S. Pat. Nos. 5,743,903, 5,810,802, 5,759,182 and 5,643,255. However, to the best of applicant""s knowledge, such devices do not address the need for a simple positioning system for a cryogenic treatment catheter.
Accordingly, there remains a need for a cryocatheter tip construction that effectively determines, reports or controls tissue contact.
There is also a need for a cryocatheter construction that more effectively cools contacted tissue.
There is further a need for a cryocatheter positioning system which is controllable to apply cooling to a predetermined tissue region.
One or more of the foregoing desirable objects are achieved in accordance with embodiments of the present invention by a cryocatheter for treatment of tissue wherein the tip of the catheter is adapted to provide a signal indicative of the quality and/or orientation of the tip contact with surrounding tissue. In one embodiment, a signal conductor extends through the catheter to the tip and connects to a thermally and electrically conductive shell or cap to apply a high frequency electrical signal to the region of tissue contacted by the tip. A surface electrode is mounted on the patient""s skin, and the tissue impedance path between the signal lead and the surface electrode is monitored to develop a quantitative measure of tissue contact in the cooling region of the distal tip. Preferably this measure is displayed on the screen of a catheter monitoring console. In yet a further embodiment, the outer portion of the tip is provided with a split thermally conductive jacket, and temperature monitoring sensors, such as thermistors or thermocouples, are mounted on both halves of the tip so as to sense temperature separately on two opposite sides of the catheter axis. The thermal signals are processed to indicate and display the differential temperature between the two sides of the tip, thus providing an indication of which side lies in contact with tissue.
In yet another aspect of the first embodiment utilizing a split conductive shell or cap, two separate and distinct high frequency electrical signals are applied to the two halves of the split tip. In that case, the signal received at the surface electrode is filtered into first and second frequency components, and these are processed to determine the relative strength of each signal component to provide an indication of the relative impedance of the path for each signal, and thus the console determines and displays the tissue contact orientation of the catheter tip itself. The system of this embodiment preferably utilizes a catheter which has separate cooling or refrigerant expansion or circulation chambers within the cooling tip. These may be positioned so that one chamber lies on each side of the axis at the tip region, and each is associated with its own thermistor or other sensor, and its own conductive wall portion. A controller monitors the temperature sensor or RF conduction of the signal electrode associated with each chamber, and the control console display indicates the tissue-contacting side of the catheter. This cryocatheter preferably includes a separate cooling inlet to each chamber, and a mechanism in or connected to the handle for directing the flow of coolant to one or the other chamber during active cryotreatment. The console may further include a controller to automatically control the valve to direct coolant to the tissue-contacting side.